1. Field of the Invention
The present invention relates to a device for controlling power generated in a vehicle, and in particular, to a device that controls the output voltage from a power generator mounted in vehicles such as a passenger car and a truck.
2. Description of the Related Art
In a vehicle, engine-stalling or vibration occurs when an electrical load mounted in the vehicle suddenly increases so that a rapid increase of generated power is caused, therefore increasing the torque of the power generator.
For preventing this problem, Japanese Patent Laid-open Publication No. 2006-121869 discloses a power generation controlling device that has a so-called gradual excitation control function. The gradual excitation control function suppresses the increase rate of an excitation current to a predetermined value or below.
To achieve the gradual excitation control function, the excitation is subjected to duty-control. A mean value of a driving duty (Hereafter, referred to as “FDUTY”) of an excitation drive circuit is required to be calculated and then stored as a value of gradual excitation (hereinafter, referred to “stored duty”). The FDUTY is required to be controlled so that the FDUTY does not exceed the stored duty. In this case, the maximum increase rate of the excitation current is determined by the increase rate of the stored duty. In the above-described conventional power generation controlling device, the maximum increase rate of the excitation current is changed depending on the status of the vehicle, and the increase rate of power generation is set to an optimal value when the electrical load is applied.
In the power generation controlling device disclosed in Japanese Patent Laid-open Publication No. 2006-121869, the maximum increase rate of the excitation current can be changed. However, the decrease rate of the stored duty is constant. In an actual vehicle, shaky rotation caused by disturbances, and fluctuations in a relatively small, periodically-operated electrical load occur during idling. Therefore, when the gradual excitation control function operates and the decrease rate of the stored duty is high, the stored duty suppresses increase in FDUTY at all times. As a result, voltage fluctuations increase depending on the shaky rotation and the fluctuations in the electrical load. Adverse effects occur, such as lamps brightening and dimming. On the other hand, when the decrease rate of the stored duty is low, such problems do not occur. However, when the electrical load is re-introduced immediately after an amount of generated power decreases after the electrical load is disconnected, the stored duty has not yet decreased. Therefore, the amount of generated power suddenly increases without a gradual excitation control operation. Accordingly, there occur problems, such as the power generator torque sharply increasing and idling speed decreasing. In this way, the gradual excitation control function is adversely affected by the electrical load state of the vehicle (i.e., variation of an amount of the electrical load), causing voltage fluctuations and rotation decrease.